In the past, in order to detect the concentration of a specific component in exhaust gas flowing through an exhaust passage of an internal combustion engine, it has been known to arrange an exhaust sensor in the exhaust passage of the internal combustion engine. The exhaust sensor is, for example, a nitrogen oxide sensor (NOX sensor) detecting the concentration of nitrogen oxide in the exhaust gas (for example, see PLTs 1 to 4).
An exhaust sensor is provided with a sensor cell having a solid electrolyte layer, a positive electrode and negative electrode. In the exhaust sensor, the current flowing through the sensor cell when a predetermined voltage is applied to the sensor cell is detected as the output of the sensor cell. Based on the detected output, the concentration of a specific component in the exhaust gas is calculated. For example, the concentration is calculated using a map showing the concentration as a function of the output of the sensor cell. In this map, the higher the output of the sensor cell, the higher the concentration.